The persistence of neutrally-to-positively buoyant and mechanically
strong lithosphere which shields the cratonic crust from underlying
mantle dynamics is generally believed to be responsible for the
longevity of cratons. In this paper it is shown by Hu et
al. that in South America and
Africa large portions of the cratonic lithosphere, however, underwent
significant modification during and following the Mesozoic, as is
demonstrated by uplift and volcanism in the
Cretaceous,
high topography of the present, thin crust and the presence of
seismically rapid though neutrally-buoyant anomalies in the upper
mantle. It is suggested by Hu et
al. that these observations reflect a permanent increase in
lithosphere buoyancy that resulted from plume-triggered delamination of
deep lithospheric roots that occurred during the Late Cretaceous and
early Cainozoic. In
these regions lithosphere has been re-established since then, as has
been confirmed by its low heat flow of the present, high seismic
velocities and realigned seismic anisotropy. Hu et
al. concluded that the
original lowermost cratonic lithosphere is compositionally denser than
the asthenospheric mantle and when perturbed by underlying mantle
upwelling can be removed. It is the buoyancy of the upper lithosphere,
therefore, that perpetuates the stabilisation of
cratons.

The stability and persistence of cratons, which are the long-lived
portions of continents that are relatively stable, has been the focus of
sustained research over the past 4 decades. According to the traditional
hypothesis of isopycnicity lithospheres of cratonic mantle are highly
depleted as a result of extensive melt extraction (1-4), which results
in a highly-viscous thermal boundary layer that persists for billions of
years that is neutrally to positively buoyant. Though this model is
elegant, it does not easily explain apparent temporal variations in
cratons, which include significant changes in elevation over time and
episodic destruction of the deep lithosphere (3,4). It is suggested by
recent measurements of seismic anisotropy that the upper portion, at
<100-150 km depth of the cratonic lithosphere has a seismic fabric that
differs from the underlying lithosphere (5-7). According to Hu et
al. this new lithospheric
structure is consistent with models that assume a compositionally
stratified thermal boundary layer in which shallower depths are more
depleted (8,9). Questions about the buoyancy and stability lithosphere
are sparked by such models (2-4), especially with regards to density
resulting from composition and temperature at different depths (8-10).
Consequently, questions that concern how cratons remain stable for
billions of years and their response to dynamic processes in the
underlying asthenosphere are yet to be answered.

In this study Hu et al.
investigated the density structure and temporal evolution of the
cratonic lithosphere by the use of diverse observational constraints,
which included topographic evolution, gravity anomalies, seismic
tomography and anisotropy, tectonic reconstructions and mantle flow. In
order to avoid the potential effects of subduction in the
Mesozoic-Cainozoic on
the evolution of the craton (2,11,12), they focused on cratons that
border the passive margins of the South
Atlantic.
Precambrian
regions of South America and Africa which host cratons and orogens from
the Neoproterozoic (13) were included in the study area, regions that
had in the past been part of western Gondwana. A prominent feature of
these is that they have widespread surface topography of 1 km elevation
or more either in the inside or on the edges of the São Francisco (SF),
Congo (CG) and Kalahari (KH) cratons. In these regions the elevation is
in sharp contrast to that of the other cratons such as the West African
(WA), Amazonian (AZ) and North American (NA) cratons that are mostly of
low topography (14,15).

Hu et al. say several key
aspects related to the topographic evolution of the study area:

1)
The highest topography in the region is supported isostatically by
sub-crustal lithospheric mantle.

2)
This high topography developed during and following the Late Mesozoic,
as is indicated by the presence of earlier marine-lacustrine
depositional environments (16,17) and of volcanic eruptions and
associated surface uplift in the Cretaceous (20-22) in these regions.

3)
Beneath the study area the upper mantle contains anomalous zones which
have high seismic velocity, though they are close to being neutrally
buoyant, which is a feature that is characteristic of delaminated
lithospheric mantle in cratons.

4)
Below these regions seismic anisotropy of lower cratonic lithosphere
showing realignment to directions to the the mantle flow in the
Cainozoic.

When taken together, the analyses of Hu et
al. indicate that the western
Gondwana cratonic lithosphere was modified by delamination that began in
the Late Mesozoic, as a result of its interaction with mantle plumes.

Origins of high cratonic topography

This crust, that is relatively thin, cannot compensate for high
topography, and therefore must reflect positive buoyancy from the
underlying mantle. Density differences between cratonic and non-cratonic
crust also cannot explain the topographic variation that is observed, as
the high topography occurs within only part of the CG craton.

In order to gain further insight into the variation of the topography,
Hu et al. estimated the
lithospheric residual topography by removing contributions of the crust
and the sublithospheric mantle, neither of which can explain the high
surface topography.

As well as the lack of correlation of the crustal thickness with
topography, as has been discussed above, neutral-to-negative dynamic
topography has been revealed in the study region by recent geodynamic
studies (28-30), a result that was confirmed by the calculations of Hu
et al. that used recent
tomographic images. As a consequence, the lithospheric residual
topography displays a peak-to-trough variation of up to 2 km, with the
positive areas correlating with the topography of the prominent land
surface. Most of the residual topography is present inside the cratons,
with only a minor portion over the Neoproterozoic (Braziliano or Pan
African) orogens. It is suggested by this result that high cratonic
topography, in regions where the crust is thin, reflects heterogeneities
in lithospheric buoyancy. This conclusion is supported further by a
similar analysis of the gravity field, with negative lithospheric
gravity anomalies occurring in regions of high topography.

Hu et al. examined seismic,
heat-flow, and xenolith data, in order to evaluate whether the
heterogeneity of the lithospheric buoyancy that is inferred has a
thermal or compositional origin. It is indicated that cold lithosphere
extends down to depths of more than 200 km (34,37)by the presence of high shear velocities(31-36), low attenuation (37) and cold geotherms that are
inferred from heat flow (38) beneath both regions of high topography of
KH, CG and SF and the 2 cratons of low topography of WA and AZ. Thermal
thickness of lithosphere is inferred from shear velocities (34) vary by
less than 25 km among the cratons KH, CG, SF,WA AND AZ.

Also, residual topography does not consistently correlate with regional
variations in thermal buoyancy of the mantle that is inferred by
attenuation (37), either within or between the cartons. The thermal
thickness of the lithosphere that is inferred from heat flow (38) varies
by less than 100 km, with cratons of high topography being slightly
thinner. Similar to slightly higher (by less than 150 k) temperature
beneath the HK craton compared to intact cratons such as the Slave, are
revealed by estimates of lithospheric temperature from mantle xenoliths.
The associated thermal buoyancy could account for less than 500 metres
of topography, which contrasts sharply with the variation that is much
larger, up to 2 km, of lithosphere residual topography, when an upper
limit of 150 K of these temperature variations that are inferred within
the lower 100 km of the lithosphere is taken. Therefore it is concluded
by Hu et al. that a
significant fraction of the apparent lithospheric buoyancy reflects
composition.

Uplift in the Cretaceous due to lithospheric delamination

Hu et al. also evaluated the
temporal evolution of cratons in order to understand better the origin
of high topography cratons. Since the Cretaceous the SF craton has
gained more than 1 km of surface elevation, relative to the adjacent AZ
craton (16). As a result, shallow marine to lacustrine sedimentary
strata dating from the Late Jurassic to Early Cretaceous in eastern
Brazil have been elevated to well above sea level (16). According to Hu
et al. most of the uplift
probably occurred during the Late Cretaceous associated with denudation
of more than 3 km in eastern Brazil; with the greatest denudation
occurring along the northeastern edge of AZ (16) and over the entire SF
(20,21), regions that have positive residual topography at the present.
Similarly, southern Africa, which is a region that had undergone
subsidence during the Jurassic, as is indicated by the presence of
sedimentary deposits that date to this age (17) – experiencing rapid
uplift and denudation during the Cretaceous, as has been revealed by
kimberlite diatremes (19), by thermochronology studies of the KH (22),
and by the evaluation of the history of sedimentation along the southern
coast of Africa (39).

It is not considered to be likely that dynamic topography due to
sublithospheric convection was the cause of uplift during the Cretaceous
and high residual topography of the present, for 2 reasons.

1)
Subsidence, not uplift, in Eastern South America (28,29), has been
revealed by recent estimates of changes in dynamic topography that is
due to mantle convection since 100 Ma. Similarly, little uplift took
place in southern Africa during the Cainozoic (28,29).

2)
Residual topography that has been observed in the study area has a
wavelength of less than 500 km, though it is usual for dynamic
topography to occur a wavelength of more than 1,000 km.

Hu et al. also examined the
possible role of lithospheric delamination in increasing buoyancy of
cratonic lithosphere, because delamination can cause isostatic uplift,
as dynamic topography in unable to explain uplift that has been
observed. During the Late Cretaceous a rapid increase in heat flow in
South America and southern Africa (19,21), and contemporaneously, the
disappearance of high-pressure garnet facies in South America (21), has
provided independent evidence of the removal of the lowermost
lithosphere. Also, the temporal correlation among the 2-pulse history of
kimberlite eruption with a composition that is more fertile from the 2nd
phase (40), and the sedimentation rate circum-Africa (39), and the
unroofing history of KH, further support the regional modification of
southern African lithosphere (40).

According to Hu et al. mantle
upwelling in
plumes
in regions that are far from
subduction zones
represent the most likely candidate for the triggering of delamination
of lithosphere. It was found by reconstructing the trajectory of
Atlantic hotspots (41) back to the Early Cretaceous by the use of recent
plate reconstruction (43), that hotspot tracks from the Cretaceous
coincide with regions where the topography is high in South America and
southern Africa. The matching of hotspots with volcanism that dates to
the Late Cretaceous in Brazil and South Africa is an independent
validation of this reconstruction. It is implied by this correlation
that there is a potential causal relationship among these phenomena. The
most intense kimberlitic volcanism from the Late Cretaceous in the SF
and KH cratons primarily occurred along their edges, which possibly
implies the location of initial lithospheric delamination. In southern
Africa thermochronology studies reveal there was a migration of
exhumation from the edge to the interior of KH during the timespan 90-60
Ma (22), which is consistent with a progressive peeling off of cratonic
lithosphere towards the centre.

A local increase in buoyancy and surface uplift can result from warming
of the lithosphere and delamination of high density materials. It is
suggested by the coherently fast seismic velocities of the thick
lithospheres (31-36) and the low surface heat flow of the present (38)
that thermal perturbation that was caused by the plume-lithosphere
interaction in the Cretaceous has now largely dissipated. This
disappearance is consistent with the time that would be required for the
delaminated lower lithosphere to be replaced by a new thermal boundary
layer. The high topography of the present must, therefore, reflect a
permanent loss of a compositionally dense layer from the original
lithosphere.

It has been revealed by 3-D representation of mantle seismic structure
that there are multiple voluminous anomalies of high velocity within the
sublithospheric mantle below the margins of the south Atlantic. It seems
these mantle structures are well resolved, as they are similar among
different topographic models. These anomalies could not represent slabs
that have been subducted as subduction has not occurred in these regions
since the early Mesozoic. They were previously interpreted and cold
downwelling along lithospheric margins, as a result of edge-driven
convection (43). It is suggested by Hu et
al. that instead they
represent the foundered segments of lithospheric mantle.

A separate study in which Hu et
al. evaluated quantitatively the mantle flow associated with the
subduction of the Nazca Plate relative to other anomalies of high
velocity by matching simultaneously predictions of mantle flow to
observations of surface wave anisotropy and shear wave splitting (44),
supported this concept. Fits to regional and local anisotropy
measurements require the Nazca slab to be the dominant cause of
downwelling and that other high-velocity anomalies are mostly drifting
passively and generating negligible vertical mantle flow. The
upper-mantle anomalies of the south Atlantic are required by this result
to be close to neutral buoyancy.

According to the model of Hu et
al. (44), the mantle underlying an overriding plate, such as South
America, undergoes Poiseuille flow (pressure driven) that is driven by a
pressure gradient. Beneath plates that are not close to a subduction
zone, in contrast, mantle undergoes plate flow that is driven by Couette
flow (drag driven). As a consequence, mantle structure of the present
further supports the lithospheric delamination hypothesis in that the
geometry and the location of these anomalies of high velocity are best
explained as fragments of foundered lithosphere left behind by South
America and Africa as they moved away from each other since the
Cretaceous. The stronger westward upper-mantle Poiseuille flow drawn by
the down-going Nazca slab (44) may be reflected in the tendency for the
delaminated materials to remain on the South American side of the
Atlantic.

Cainozoic realignment of lithospheric seismic anisotropy

Seismic anisotropy that is similar to that of recent asthenospheric flow
should be displayed by lithosphere that has undergone shear during the
Cainozoic. Another important test of the model of Hu et
al. (44) can be provided by
comparison of azimuthal anisotropy (6) with that of what is predicted by
Cainozoic mantle flow, as a function of depth. Robust predictions of
anisotropy within the asthenosphere underlying continental South America
and the surrounding oceans allows the application of modelling results
to the interior of the cratonic lithosphere.

It is suggested by the lack of correlation between the observed
anisotropy in the uppermost <100 km of the mantle with that of the
asthenosphere that fabrics at this depth are mostly ‘fossil’ signals,
meaning relicts of shear prior to the Cainozoic. The correlation clearly
improves in most non-cratonic regions at depths of ≥100 km, the
correlation clearly improves in most non-cratonic regions, which is
consistent with their thin lithospheres and an increasing influence of
shear in the Cainozoic. Anisotropy that is compatible with shear from
the Cainozoic at greater than 150 km depth is not displayed by the
region of the East African Rift, which Hu et
al. suggest is probably
because of perturbations by upwelling of the upper mantle. Contrasting
with this, anisotropy correlation is poor below the low topography
cratons, which are inferred to have intact mantle lithospheres (AZ and
WA). It is emphasised by this lack of correlation that deformation of
the mantle did not occur within the intact cratonic lithosphere during
the Cainozoic (5,6).

In most cratonic regions of high topography, however, anisotropy that is
observed aligns with mantle shear that has been predicted at depths as
shallow as 100 km, such as in SF. In cratonicregions with the most high topography, however, anisotropy that
is observed aligns with mantle shear that has been predicted at depths
as shallow as 100 km, such as in SF. When a different anisotropy model
(45) was used there was a similar correlation of anisotropy that is
observed and predicted results. It is suggested that the lithospheric
fabrics of these cratons were reset by this obvious regional realignment
of lower lithosphere anisotropy with mantle flow in regions where there
was modification during the Mesozoic. This conclusion was confirmed by
another anisotropy model (46) for depths of less than 150 km, though at
greater depths significant discrepancy is displayed. As this model fails
to match their asthenospheric deformation, that is well-predicted, and
probably also fails to represent the shear wave-splitting observations
over South America (44), Hu et al.
suggest that the other 2 anisotropy models (6,45) represent better
lowermost lithosphere fabrics in the study area.

Mechanically, resetting of fabric could be the result of either local
deformation of the original lithosphere (11) or growth of a new thermal
boundary layer following delamination. Hu et
al. suggest a combination of
both mechanisms. Isostatic uplift of topography, on the one hand,
requires dense lithospheric materials to be removed. On the other hand,
the high velocity (more than 2 %) lithosphere at 200 km and below
requires compositional (e.g. garnet or MORB) as well as their thermal
effects. Hu et al. therefore
propose that delamination occurred mostly within the lowermost cratonic
lithosphere where there are high density materials, and that lithosphere
was not necessarily removed entirely below about 100 km, instead
undergoing shear that is sufficient to realign it crystallographic
anisotropy with that of the ambient mantle. It is suggested by Hu et
al. that this concept can be
verified further by an examination of the spatial extent of anisotropy
realignment, where the area in Africa that is realigned is much larger
than the zones of high residual topography, with the former also
correlating closely with the province of kimberlites from the Cretaceous
(47).

Implications for the density and evolution of cratonic lithosphere

The density of the lowermost cratonic lithosphere must be greater than
that of the surrounding asthenosphere for it to delaminate. Also, as the
craton topography of the presence in the study area rises significantly
higher than the pre-delamination topography that is reflected in uplift
that occurred in the Cretaceous and present positive residual
topography, the lithosphere must have also been denser than the thermal
boundary layer that formed subsequently. According to Hu et
al. this proposal explains
why ‘intact cratons’, such as WA and AZ, have topography that is lower
than that of ‘delaminated cratons’, e.g. SF, CG and KH. The proposal of
Hu et al. contrasts with the
traditional view that cratonic roots have approximately buoyancy at all
depths (1,3,4). Instead, Hu et al.
proposed that in cratons the upper lithospheric mantle is highly
depleted and chemically buoyant, possibly even more so than has yet been
inferred from xenoliths in the mantle (4,40). The lower lithosphere in
cratons is more fertile, and grades downwards into a boundary layer that
is purely thermal, which contains zones or layers at more than 200 km
depth enriched in high density minerals.

The model of Hu et al. is
consistent with the occurrence of garnet-peridotite in lower portions of
cratons that are intact, such as Slave and KH cratons that are from
pre-Cainozoic times (3,4,21,40), and with high seismic velocities in the
average lowermost 150-200 km cratonic lithosphere (35). Assuming a
thickness of 50 km of this high-density layer, 20% excess
garnet-peridotite relative to a lowermost lithosphere that is neutrally
buoyant are required by the residual topography and gravity, if the
density of garnet-peridotite is about 10% higher than that of peridotite
(4). Hu et al. proposed that
this high-density layer may have been emplaced during the formation of
the craton (48) and/or represents secular accumulation of
basaltic melts from the
deep mantle (49).

The lithosphere-density model of Hu et
al. also explains several
enigmatic properties of the prominent high velocity seismic anomalies of
the upper mantle below the south Atlantic. Included among these are
anomalies in regions that are far from subduction zones. Notably,
delaminated lithosphere that consists of dense garnet-peridotite and
buoyant harzburgite should have small initial negative buoyancy with a
magnitude that would also decrease with increases of depth and
temperature. Within the warming delaminated root at a depth of 660 km a
delayed post-garnet transformation would generate additional positive
buoyancy and prohibit further penetration into the lower mantle. The
stagnation of the delaminated materials at the base of the upper mantle
is explained:

1)
First, by this concept, and

2)
second, their minimal net buoyancy, therefore avoiding the generation of
local convection, as is required by seismic anisotropy.

3)
Finally, the high seismic velocities of these anomalies are consistent
with the enrichment of harzburgite and garnet at temperatures that are
relatively low.

It was noted by Hu et al.
that the minimum depth of about 100 km of the anisotropy realignment
correlates broadly with the mid-lithospheric discontinuities (MLDs) that
are commonly present beneath continents (26,51-54). It is implied by
this correlation that the MLD may represent a layer that is mechanically
weak inside the lithosphere, below which shear deformation of more than
100 km and ultimately delamination at greater than 150-200 km depth
could occur more easily than in the layer of lithosphere above. Hu et
al. suggest this conclusion
is consistent with recent suggestions that the MLD represents the top of
a weak layer of solidified volatile-rich melt accumulation (49,55). A
decoupling surface between the upper and lower lithosphere could be
served by such a weak layer during extension (56), thereby facilitating
mobilisation and delamination of dense deep lithosphere.

The results presented in this paper, when taken together, indicate that
lowermost cratonic mantle lithosphere may be removed episodically when
mantle processes perturb them sufficiently, while the buoyant
lithosphere that is depleted tends to remain stable with the result that
its crust maintains cratonic characteristics over geological time. The
key features of the destabilisation of cratonic lithosphere, during
which deformation and removal of the dense root facilitates kimberlite
volcanism (18-22), isostatic uplift of the surface, and shallow Moho and
thin crust. It is further proposed by Hu et
al. that zones in which lower
lithosphere was removed takes 10s of millions of years to recover
thermally, though the density of the new thermal root would be less than
that of the intact root. Cratons without hotspot activity in the
Phanerozoic
and subduction that is nearby, such as those in the Northern Hemisphere,
have not undergone delamination since the Mesozoic and therefore display
negative residual topography and mantle gravity anomalies that are
positive, which contrasts with cratons that are affected by subduction,
such as those in East Asia (11) and western North America (12), and
cratons that are affected by mantle plumes, such as those in Southern
Hemisphere.